Process for making a sintered photoconductive body

ABSTRACT

A process for making a sintered photoconductive body comprising growing cadmium sulfide crystals in the presence of hydrogen chloride gas to provide donor doping of the resultant crystals, pulverizing the donor crystals and measuring the chlorine content of the resultant pulverized donor doped material, mixing a quantity of copper with the donor doped material to provide acceptor doping, the quantity of copper being in a selected ratio to the quantity of chlorine, and finally, sintering the mixture to form the sintered photoconductive body.

United States Patent 1191 Marlor et al.

1451 Aug. 28, 1973 PROCESS FOR MAKING A SINTERED PHOTOCONDUCTIVE BODY [75] lnventors: Guy A. Marlor, Los Altos; John B.

Mooney, Saratoga, both of Calif.

[73] Assignee: Photophysics, Inc., Mountain View,

Calif.

[22] Filed: Apr. 5, 1971 [21] Appl. No.: 130,983

3,222,215 12/1965 Diirr 117/106 R FORElGN PATENTS OR APPLICATIONS 1,001,443 8/1965 Great Britain 1 17/201 Primary Examiner-Alfred L. Leavitt Assistant Examiner-M. F. Esposito Attorney-Lowhurst & l-lamrick [57] ABSTRACT A process for making a sintered photoconductive body comprising growing cadmium sulfide crystals in the presence of hydrogen chloride gas to provide donor doping of the resultant crystals, pulverizing the donor crystals and measuring the chlorine content of the resultant pulverized donor doped material, mixing a quantity of copper with the donor doped material to provide acceptor doping, the quantity of copper being [56] References Cited UNXTED STATES PATENTS 1n a selected ratio to the quantity of chlorine, and finally, smtermg the mixture to form the sintered photo- 3,598,645 8/1971 Winter 117/201 conductive body 3,373,059 3/1968 Augustine 136/89 3,133,888 5/l964 Oikawa et al 252/501 11 Claims, 2 Drawing Figures A r LIGHT t s 2 U D 2 O C D 0 8 l J Los DONOR-TO-ACCEPTOR RATIO Patented Aug. 28, 1973 IGHT {L LOG DONOR-TO-ACCEPTOR RATIO DARK Fig-1.

C1, CONCENTRATION vs. HCJ FLOW HCI; cc/min L I 3'0 40 so 60 M00 \m20. 0- x ZOCRmhZMOZOU Q0 Fig-2 ATTORNEYS PROCESS FOR MAKING A SINTERED PHOTOCONDUCTIVE BODY BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to photoconductive devices and more particularly to a process for making a sintered photoconductive body, such process enabling the photoconductive characteristics of the body to be closely controlled and selectable with a high degree of reliability.

2. Discussion of the Prior Art Photoconductive devices are generally comprised of a body of photoconductive material sandwiched between a pair of electrodes and have the characteristics that with a voltage applied across the electrodes the device displays a decrease in electrical resistance proportional to the intensity of the radiation incident thereupon. in similar devices used in xerography and other electrostatic image producing systems, however, only one electrode is typically utilized with the electrostatic image being developed on an opposite surface of the body. In either case, the amount of current flowing through the device is a function of the electrical resistance of the photoconductive body, and in turn, the electrical resistance of the body is a function of the incident radiation due to the characteristics of the photoconductive material.

The ideal photoconductor would therefore have infinite resistance in the absence of light and be a good conductor in the presence of a maximum intensity of light to which it is sensitive. in actuality, however, a photoconductive device behaves like a high resistance conductor when light to which it is sensitive is absent, and behaves as a lower resistance conductor when light to which it is sensitive is present. The current passed by the device in darkness is typically referred to as the dark current while the current passed when the device is irradiated is referred to as the light current. The difference between the light current to the dark current is usually referred to as the photocurrent.

Although there are several types of photoconductive devices which exhibit the above mentioned characteristics, one type which is of particular interest in the field of electrophotography is the sintered photoconductor. As disclosed in the US. Pat. to Thomsen No. 2,765,385, such photoconductive layers are usually made by forming a stratum including particles of a material selected from the group consisting of sulfides, selenides, and sulfoselenides of cadmium; recrystalizing the material in a molten solvent to a desired range of particle sizes; incorporating into the recrystalized material activator proportions of a halide and activator proportions of a metal selected from the group consisting of copper and silver; and then evaporating the molten solvent to produce a substantially continuous layer of interlocked crystals of photoconductive material. In such processes the volatility of the constituents is relied upon to remove excess donor ions until about equal portions of donor, and acceptor ions exist.

It is known that one of the controlling factors in determining the light and dark conductivities of photoconductive bodies is the atomic ratio of the donor-toacceptor ions in the body and that a material having a high donor-to-acceptor ratio is most suited for high photographic speed applications even though it has a high dark conductivity. On the other hand, a material having a low donor-to-acccptor ratio is most suited for low photographic speed applications since it has a low dark conductivity. Although the light to dark conductivity characteristics at the ratio extremes, i.e., at ratios greater than four and less than one, change relatively little with changes in donor-to-acceptor ratios, there is a band of ratios (ratios between one and four) over which a more rapid transition occurs in the dark conductivity than in the light conductivity and having the ability to control the ratios in this band permit the manufacturing of photoconductors having selectable characteristics which substantially span the spectrum of photoconductor characteristics. Reliable control of the ratios in this band has heretofore not been available.

In such processes as disclosed in the aforementioned Thomsen patent, the volatility of the constituent is relied upon to remove excess donor ions until about equal proportions of donor and acceptor ions exist. This is a relatively straight forward process since donor ions will be liberated fairly easily until their proportion is approximately equal to that of the acceptor ions (a ratio of one) andthe volitalization will for all practical purposes stop when this level is reached. The use of this type of process to provide a donor-to-acceptor ratio greater than one is quite unreliable since the precision of the process is largely dependent upon stopping the volitalization at some precise stage of the process with such stage being primarily a matter of cut and try.

It is also advantageous to have a process available capable of providing donor-to-acceptor ratios of less than one since at such ratios a very low dark current characteristic is obtained. Very low dark current characteristics are desirable for electrophotographic applications requiring low light levels and long exposure times. Prior art processes do not permit the provision of ratio substantially less than one since as indicated above the presence of the acceptor ion during the volitalization of the donor tends to prevent the liberation of donor ions once a balanced condition exists.

SUMMARY OF THE PRESENT INVENTION It is therefore an object of the present invention to provide a process for making a photoconductive body which permits close control of donor concentration and donor-to-acceptor ratio and thereby enables accurate control of the photoconductive characteristics of the body.

Broadly stated, the method of the present invention includes the heating of a compound of materials selected from Groups II and VI of the Periodic Table of the Elements in a gas containing a halogenous donor element in order to grow donor doped crystals; analyzing the donor doped crystals to determine the quantitative content of the halogenous donor element; mixing a selected quantity of acceptor metal, such as copper, silver, or gold, with the donor doped crystals sufficient to produce the desired atomic ratio of donor-toacceptor; and then sintering the mixture to form the sintered photoconductive body.

Among the principal advantages of the present invention is that the respective quantities of donor and acceptor elements can be accurately and selectively matched, i.e., the atomic ratio of donor-to-acceptor can be closely controlled, and accordingly, particular light and dark conductivity characteristics can be reliably repeated in a large number of photoconductive devices.

Another advantage of the present invention is that it enables precise control of the concentration of donor and acceptor materials in a particular photoconductive body.

These and other advantages of the present invention will no doubt become apparent to those skilled in the art after having read the following detailed disclosure.

IN THE DRAWING FIG. 1 is a logarithmic plot of conductivity versus donor-to-acceptor ratios for sintered photoconductive bodies.

FIG. 2 is a plot of chlorine concentration versus hydrogen chloride gas flow as utilized in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT As mentioned above, a photoconductive device has one resistance in intense light and another resistance in dark (more resistance in the dark and less resistance in the light) and by controlling the atomic ratio of donorto-acceptor elements in the photoconductive material, the relative light and dark conductivities, or conversely the relative light and dark resistivities can be selected with a high degree of accuracy. If a given photoconductive material contains excess donor ions, the material will have good conductivity in both light and dark, however, if there are excess acceptor ions, both light and dark conductivities will be reduced, but the dark conductivity will be reduced substantially more than is the light conductivity. This is indicated by the diagram shown in FIG. 1 of the drawing.

In FIG. 1, a light conductivity curve and a dark conductivity curve are shown generally illustrating respectively, the saturated light conductivity, and the dark conductivity of a particular type of photoconductive body as a function of the atomic ratio of donor-toacceptor ions in that type of body. It will be noted that although both light and dark conductivities decrease with decreasing donor-to-acceptor ratios there is a particular range R of ratios over which the dark conductivity decreases at a substantially higher rate than does the light conductivity. This conductivity-to-doping ratio diagram clearly suggests that in order to obtain selectivity of photoconductor characteristics and reliable repeatability of manufacture, one must use a process in which the donor-to-acceptor ratio can be closely controlled. Such a process would permit the selective production of photoconductive devices having particular characteristics that are suited for particular applications.

For example, for high speed electrophotographic applications, the dark conductivity characteristics is of little consequence and maximum light conductivity is desired, therefore a photoconductive layer at the higher ratio end (ratios 2 2) of the range R would be preferred since the light conductivity is greater at that end of the range. However, for low speed electrophotographic applications wherein long exposure times are required, it is more desirable to use a donor-toacceptor ratio toward the lower ratio end (ratios l) of the range R so as to avoid excessive picture fog or noise." In other words, the right hand side of range R may be thought of as having a high signal-to-noise ratio whereas the left hand side of range R can be thought of as having a low signal-to-noise ratio. In general, as the donor-to-acceptor ratio is decreased, the time required for the photoconductor to respond'to light increases. This slowing response characteristic is usually undesirable since optimum operation is obtained with the highest donor-to-acceptor ratio possible consistent with a permitted fog or noise standard.

However, it should also be pointed out that for some very low speed electrophotographic applications it is desirable to use photoconductors having a donor-toacceptor ratio of substantially less than one. Such ratios were very difficult to obtain using prior art methods since the presence of the acceptor ions will prevent the loss of the donor ion by volitalization below a ratio of one. The difference in conductivity characteristics of a photoconductive body having a ratio of one and a body having a ratio of much less than one may be illustrated (see FIG. 1) by comparing the decrease A in light conductivity (below the level at the high ratio end of the spectrum) and the decrease B in the dark conductivity at a ratio of one to the decrease C in light conductivity and the decrease D in dark conductivity at a ratio of much less than one. In other words, the signal-to-noise ratio of a photoconductor having a donor-to-acceptor ratio of one can be expressed as A/B and the signal-tonoise ratio of a photoconductor having a low donor-to acceptor ratio (much less than one) can be expressed as C/D. The difference is quite apparent.

The present invention relates to a process wherein a donor ion, such as chlorine (Cl), is controllably introduced into a growing or recrystallizing crystal lattice, such as of cadmium sulfide (CdS), in a first step, and then after actual quantitative chemical determination of the donor level in the resultant crystals, the precise quantity of a material containing an acceptor ion, such as copper (Cu), which is necessary to produce a selected donor-to-acceptor atomic ratio (that will provide the desired photo response) is mixed with the donor doped crystals. The donor ions in the resultant layer provide electrons through the conducting lattice which are electrically compensated for by the subsequently added acceptor ions. The mixture is then pulverized, disposed over a suitable substrate by painting or spraying, and subsequently sintered without a solvent (a solventless sintering) to produce a more dense layer than is achieved with the solvent sinter of the prior art.

The advantages of the more dense sinter include (1) the lessened need for surface protection or encapsula' tion and (2) a more efficient structure for certain applications.

Whereas, in the prior art, a compound containing the donor ion is mixed with a compound containing the acceptor ion, for example, chlorine is put in as copper chloride, in accordance with the present invention, the donor ion is put into the cadmium sulfide (or other compound) in a gas phase and the acceptor ion is later added after it is known precisely how much donor ion is present. Since the donor ion quantity is first determined, a very accurate determination can be made of the amount of acceptor ion which must be added to give a particular donor-to-acceptor ratio. And since the amount of donor ion present in the resultant body is independent of the later added acceptor ion, any desired ratio may be obtained, even ratios of less than one.

The fact that the crystal is grown (or recrystallized) in the vapor of the donor ion assures that the donor ion is not only present in a certain quantity but is also distributed throughout the crystal lattice unifonnly. One of the key features of the present invention is therefore the growing of the photoconductive crystals in the donor ion containing vapor so that the donor ion appears within the crystal structure rather than being associated with the surface of the very fine powders that are produced. In the above mentioned copperchloride doped layer, the chlorine molecules are believed to either take the place of a sulfur atom in the lattice (substitutional disposition), or to be dispersed in between sulfur atom (interstitial disposition).

The following more specific example is illustrative of the process of the present invention.

EXAMPLE Donor Introduction 100 grams of cadmium sulfide (CdS) is placed in a quartz boat in a 3 inch ID quartz tube and argon gas is initially passed through the tube at a rate of 500cc per minute to purge the tube of other gases. With the argon gas flowing HCI gas is then introduced at 6000 per min ute and the tube is heated to 1,200C in 1.5 hours, held at that temperature for 3 hours and then cooled to room temperature with the argon and the HCI gas flowing. During the 1,200C stage, sublimation of the CdS occurs with the result that crystals of Cl doped CdS are grown. The substantially saturated donor (1) doped crystals are then removed from the tube and pulverized by ball milling.

Donor Measurement The donor content of the doped CdS is determined by a measurement of the turbidity produced when a solution of AgNO is added to a sample of the pulverized crystals which has been dissolved in HNO Acceptor Introduction The pulverized crystals are then treated with a predetermined quantity (that necessary to produce the desired donor-to-acceptor ratio) of acceptor material (Cu) by suspension in water containing the required quantity of copper as CuSO .5I-I,. A binder material such as solder glass (Coming 7570) is then added in quantity equivalent to grams glass per 100 grams of CdS.

Layer Preparation and Sintering The mixture is then again ground by ball milling, a layer of the ground mixture is applied by spraying onto a Pyrex plate glass having an electrically conducting surface coating of tin oxide, and the layer is then sintered by firing for 35 minutes at 550C.

Although not required, a post firing aging treatment at 80C for 60 hours is useful in producing a stable photoconductor. The elevated temperature rapidly brings the crystal surfaces into equilibrium with the ambient temperature and air. This aging process is common in the photoconductor art (see Corrsin US. Pat. No. 3,288,604).

In FIG. 2, a graphical relationship between flow-rate of HC1 and C1 concentration is presented which can be used as a guide to control the C1 concentration in the growing crystals. Although this relationship holds in general, it is to be appreciated that absolute magnitudes will vary with the operating conditions and apparatus used. The diagram does, however, indicate that for a given apparatus and set of conditions the CI concentration established in the growing crystal can be controlled by selection of the HCI flow rate.

The temperature of 1,200C is used in the above example so that the crystal is transported (or sublimed) down the growth tube in vaporized form to a point lower in temperature where the CdS condenses and recrystallizes with the resultant crystal lattice containing the CI. The CI has also been introduced at lower recrystallizing temperatures (such as 950C) without sublimation with similar results being obtained. The relative merits of temperature range have not thus far been established.

The temperatures and times used for the sintering step fall within the ranges of 525C to 650C for periods of 60 minutes to 10 minutes. I

Although cadmium sulfide is used in the above example, zinc, selenium, tellurium and possibly other elements from the II and VI Groups of the Periodic Table of the Elements can be substituted in whole or in part for the cadmium and sulfur without substantially differing results, although optimum conditions of temperature, time and donor-to-acceptor ratio may change. The chlorine and copper elements used in the above example are preferred as donor and acceptor elements. However, iodine has been used and it is known in the art that fluorine and bromine are also useful donors and silver and gold are useful acceptors.

To illustrate results obtained using the process described above, three samples of donor doped CdS were selectively doped in different amounts with the acceptor Cu to obtain sintered layers having different Cl/Cu ratios. These ratios and the relative proportions of Cl and Cu in the samples are shown in Table I.

TABLE I CI Cu Samples HCI g atoms] g atoms/ Cl/Cu cclmm g CdS g CdS ratio A 55 9.7 3.0 3.18 B 55 9.7 3.9 2.47 1

C 55 9.7 4.8 2.00 55 9.7 9.7 1.00 A 45 6.1 3.0 2.02 B 45 6.1 3.2 1.87

C 45 6.1 4.4 1.37 D 45 6.1 6.1 1.00 A 24 3.5 3.0 1.14 B 24 3.5 11.8 0.30

The photoresponses of these singered layers were then determined using an unfiltered quartz iodide light source. The resultant data is presented in Table II. The light flux was 3.6 X 10 photons/cml sec.

TABLE II Example Cl/Cu 1,,

A 3.18 1.9 X 10 2.5 X10" B 2.47 6.1 X 10" 6.0 X 10'' C 2.00 2.8 x10 31x10" 1.00 5.1 X 10 4.0 X 10" A 2.02 1.5 x10 1.5 x 10" B 1.87 5.7 X 10" 5.0 X10" 1.37 1.8 X 10 3.1X10" 1.00 4.7 X 10' 3.0 X 10" A 1.14 3.2x10* 1.5Xl0" B 0.30 4.2 X 10" 1.2 X 10*" 0.21 1.2 X 10' 8 0 X 10"" D 0.12 2.4 X10" 1 1 X 10 The data in Table ll substantiates the validity of the curve shown in FIG. 1 and indicates that at low donorto-acceptor ratios the photoresponse will be independent of the actual donor and acceptor concentrations over a relatively wide range.

Although a particular example including certain steps and materials has been cited above for purposes of illustration, it is contemplated that certain modifications can be made thereto. Accordingly, it is intended that the appended claims be interpreted as covering all such modifications which fall within the true spirit and scope of the invention.

What is claimed is:

l. A process for making a sintered photoconductive body comprising the steps of:

heating a compound comprised of elements selected from Group II and Group IV of the Periodic Table in the presence of a gas containing a donor element selected from the group consisting of fluorine,

chlorine bromine, and iodine to form substantially saturated, donor doped crystals of said compound; pulverizing said donor doped crystals;

determining the quantitative content of said donor element in said donor doped crystals and thereby said crystal donor level;

mixing a measured quantity of an acceptor element,

selected from the group consisting of copper, silver and gold, with said pulverized donor doped crystals, said measured quantity being determined to form an acceptor level in said crystals which pro vides a selected donor-to-acceptor atomic ratio; and

sintering the resultant mixture to provide the sintered photoconductive body.

2. A process for making a sintered photoconductive body as recited in claim 1 wherein said compound includes elements selected from the group consisting of zinc, cadmium, sulfur, selenium and tellurium.

3. A process for making a sintered photoconductive body comprising the steps of:

growing substantially saturated, donor doped crystals of a compound comprised of elements selected from Groups ll and VI of the Periodic Table of the Elements in the presence of a gas containing a donor element selected from the group consisting of fluorine, chlorine, bromine and iodine;

pulverizing and mixing said donor doped crystals with a quantity of an acceptor element selected from the group consisting of copper, silver and gold, said quantity of acceptor element being a particular percentage of the quantity of said donor element in said donor doped crystals; and

sintering the mixture to form the sintered photoconductive body.

4. A process for making a sintered photoconductive body as recited in claim 3 and wherein said compound includes elements selected from the group consisting of zinc, cadmium, sulfur, selenium and tellurium.

5. A process for making a sintered photoconductive body as recited in claim 4 and further comprising the step of determining the quantity of donor element in the donor doped crystals prior to selecting the quantity of acceptor for mixing with the donor doped crystals.

6. A process for making a sintered photoconductive body as recited in claim 4 wherein during said growing step said compound is heated to a temperature less than its sublimation temperature but sufficient to cause the compound to recrystallize.

7. A process for making a sintered photoconductive body as recited in claim 4 wherein said mixture is sintered by firing at a temperature within the range of 525C to 650C for a period of time within the range of 60 minutes to 10 minutes.

8. A process for making a sintered photoconductive body of a compound known to exhibit photoconductive effects, comprising the steps of:

selecting a donor-to-acceptor atomic ratio in accordance with a selected set of light and dark conductivity characteristics for a particular photoconductive compound;

doping a quantity of said compound with a known quantity of donor containing material to form a substantially saturated donor doped compound; and

subsequently doping said donor doped compound with a quantity of acceptor containing material commensurate with said selected atomic ratio.

9. A process for making a sintered photoconductive body as recited in claim 8 wherein the donor doping step includes heating said compound to a predetermined temperature in the presence of gaseous donor containing material.

10. A process for making a sintered photoconductive body as recited in claim 8 wherein the acceptor doping step includes mixing said acceptor containing material with said donor doped compound and sintering the resultant mixture.

1 l. A method for making a sintered photoconductive body of a compound known to exhibit photoconductive effects after being donor and acceptor doped, which comprises the steps of:

forming substantially saturated donor doped crystals of the compound in the absence of the acceptor; and

sintering the donor doped crystals in the presence of a measured quantity of acceptor, the amount of acceptor being selected in accordance with the amount of donor in said doped crystal and a selected ratio of donor level to acceptor level.

mg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,754,985 Dated Aug. zs 1973 lnvefitofls) uy A. Marlor and John B. Mooney- It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Claim 1 line 4; change "Group IV to --Group VI-- v Signed and sealed this 30th day of April 197A.

(SEAL Attest:

EDR IARDPLFLETOIEBRJR. C. MARSHALL DANN Attesting Officer Commissioner of Patents 

2. A process for making a sintered photoconductive body as recited in claim 1 wherein said compound includes elements selected from the group consisting of zinc, cadmium, sulfur, selenium and tellurium.
 3. A process for making a sintered photoconductive body comprising the steps of: growing substantially saturated, donor doped crystals of a compound comprised of elements selected from Groups II and VI of the Periodic Table of the Elements in the presence of a gas containing a donor element selected from the group consisting of fluorine, chlorine, bromine and iodine; pulverizing and mixing said donor doped crystals with a quantity of an acceptor element selected from the group consisting of copper, silver and gold, said quantity of acceptor element being a particular percentage of the quantity of said donor element in said donor doped crystals; and sintering the mixture to form the sintered photoconductive body.
 4. A process for making a sintered photoconductive body as recited in claim 3 and wherein said compound includes elements selected from the group consisting of zinc, cadmium, sulfur, selenium and tellurium.
 5. A process for making a sintered photoconductive body as recited in claim 4 and further comprising the step of determining the quantity of donor element in the donor doped crystals prior to selecting the quantity of acceptor for mixing with the donor doped crystals.
 6. A process for making a sintered photoconductive body as recited in claim 4 wherein during said growing step said compound is heated to a temperature less than its sublimation temperature but sufficient to cause the compound to recrystallize.
 7. A process for making a sintered photoconductive body as recited in claim 4 wherein said mixture is sintered by firing at a temperature within the range of 525*C to 650*C for a period of time within the range of 60 minutes to 10 minutes.
 8. A process for making a sintered photoconductive body of a compound known to exhibit photoconductive effects, comprising the steps of: selecting a donor-to-acceptor atomic ratio in accordance with a selected set of light and dark conductivity characteristics for a particular photoconductive compound; doping a quantity of said compound with a known quantity of donor containing material to form a substantially saturated donor doped compound; and subsequently doping said donor doped compound with a quantity of acceptor containing material commensurate with said selected atomic ratio.
 9. A process for making a sintered photoconductive body as recited in claim 8 wherein the donor doping step includes heating said compound to a predetermined temperature in the presence of gaseous donor containing material.
 10. A process for making a sintered photoconductive body as recited in claim 8 wherein the acceptor doping step includes mixing said acceptor containing material with said donor doped compound and sintering the resultant mixture.
 11. A method for making a sintered photoconductive body of a compound known to exhibit photoconductive effects after being donor and acceptor doped, which comprises the steps of: forming substantially saturated donor doped crystals of the compound in the absence of the acceptor; and sintering the donor doped crystals in the presence of a measured quantity of acceptor, the amount of acceptor being selected in accordance with the amount of donor in said doped crystal and a selected ratio of donor level to acceptor level. 